The present invention relates to a process for selective leaching of nickel-copper matte and especially to a process in which finely ground nickel-copper matte is leached using acid at an elevated temperature and under oxidizing conditions in order to produce a nickel sulfate solution and a copper sulfide precipitate.
Previously known is a process for refining nickel matte to high-grade cathode nickel, copper powder and cobalt hydroxy precipitate, comprising five principal stages: matte grinding, leaching, copper removal, cobalt removal, and nickel electrolysis. The Flow scheme of this known process is shown in FIG. 1.
The matte is wet ground in a ball mill. The average particle size of the ground matte passing to the leaching stage is 20-30.mu. and its chemical composition is approximately as follows: 55% Ni, 35% Cu, 1% Co, and 6% S and the sieve analysis 90% under 0.094 mm.
The leaching is performed in two or more stages countercurrently, using the anolyte of the nickel electrolysis. Mineralogically the matte consists of the following constituents: Ni-Cu alloy, nickel sulfide Ni.sub.3 S.sub.2, and copper sulfide Cu.sub.2 S. The leaching is based on the following chemical reactions: EQU Cu.sup.++ +Ni=Cu.dwnarw.+Ni.sup.++ ( 1) EQU Cu+1/2O.sub.2 =CuO (2) EQU CuO+H.sub.2 SO.sub.4 =CuSO.sub.4 +H.sub.2 O (3) EQU Ni.sub.3 S.sub.2 +H.sub.2 SO.sub.4 +1/2O.sub.2 =NiSO.sub.4 +2NiS+H.sub.2 O (4)
During the first leaching stage, the ground matte meets the solution coming from the first copper removal stage. This solution contains copper ions and sulfuric acid as active components. Reactions 1-3 continue until the copper is completely precipitated and the solution is neutralized.
After the first leaching stage, the solid material in the reactors is settled and directed to the second leaching stage. The overflow solution from the settler is filtered and directed to the cobalt removal.
During the second leaching stage, the already partly reacted matte material meets a solution coming from the second copper removal stage. As in the first leaching stage, the active constituents of the solution are copper ions and sulfuric acid. The leaching reactions occur according to Reactions 1-4. After the second leaching stage, solid material is settled and directed to the third leaching stage. The overflow solution from the settler is filtered and directed to the copper removal.
During the third leaching stage, the matte which has undergone the first and second leaching stages meets the overflow solution from the nickel electrolysis, which contains sulfuric acid 50 g/l as the active constituent. Reactions 1-4 continue until the bulk of the soluble metals (Ni, Cu, Co) of the matte has dissolved.
Sulfur is insoluble in an atmospheric leaching process and combines with part of the nickel and copper present in the matte. This part, the leach residue, is settled, filtered and returned to the nickel smelting plant. The overflow solution from the settler is filtered and directed to the copper removal.
That part of the copper which has been leached during the second and the third leaching stages is removed electrolytically. Sulfuric acid is produced as a product of the electrolysis at the insoluble lead anode, and this sulfuric acid is used for the leaching. Owing to the high cathodic current density, the copper is precipitated in powder form.
The solution from the first leaching stage is directed to the cobalt removal, where cobalt is precipitated out from the solution by using trivalent nickel (Ni(OH).sub.3): EQU Co.sup.++ +Ni(OH).sub.3 =Co(OH).sub.3 +Ni.sup.++ ( 5)
Nickel(III) hydroxide is produced by oxidizing nickel(II) hydroxide in special oxidation vessels by means of electric current. Nickel(II) hydroxide is precipitated out from the nickel electrolyte by means of sodium hydroxide. Other impurities such as copper, zinc, iron, arsenic, lead, antimony, etc. are also precipitated out from the electrolyte during the cobalt removal stage.
Owing to the use of sodium hydroxide for the precipitation of nickel(II) hydroxide, sodium sulfate is produced during the process. In order to maintain a certain sodium sulfate level, Glauber salt is crystallized out from the process solution.
The last stage of the process is the electrolytic separation of nickel in a so-called diaphragm electrolysis. The cathode is situated in a diaphragm bag into which pure nickel electrolyte is fed. The anode material is pure lead.
The electrode reactions are as follows: EQU Ni.sup.++ +2e.sup.- =Ni.dwnarw. (Cathode) (6) EQU SO.sub.4.sup.= +H.sub.2 O=H.sub.2 SO.sub.4 +1/2O.sub.2 +2e.sup.- (Anode) (7)
The sulfuric acid anodically produced during the electrolysis is used for the leaching of the matte, since the overflow solution, the so-called anolyte, from the electrolysis tanks is returned to the leaching cycle. The period of growth of the cathode from the thin start sheet to a full weight cathode (80 kg each) is about 7 days, whereafter it is removed, washed, cut and packaged for selling. The removed cathode is again replaced with a new thin nickel start sheet for growing.
This prior known process is very flexible considering the Ni-Cu ratio in the raw material, i.e. nickel-copper matte. Two comparison experiments on a pilot scale are given below. In these experiments, the described known leaching process is applied to different types of Ni-Cu mattes.